Patent GB-927437-A (GB)
Title: Optical correlation of signals
Inventor: MARGARY HARRY HERBERT
IPC: G01V1/28
Filed: 1959-09-18
Abstract: 927,437. Photo-electric record investigation. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Sept. 16, 1960 [Sept. 18, 1959], No. 31816/59. Class 40 (3). [Also in Group XXXVI] Relates to systems for cross correlating two or more electrical signals wherein each of the signals to be correlated is converted into a visible pulse record with respect to time of at least the sign (above or below a fixed datum level) of the signal at any instant and then two or more such records are optically scanned simultaneously so as to extract from them the cross correlation function of the original signals. A continuously generated signal (as shown in row 1, Fig. 1) may be limited so that only sign and zero crossing information is retained in the output from the limiter, and this output is converted into a series of alternate transparent and opaque patches on a transparent strip, length along the strip representing time. When two such records are superimposed, light will only pass through those areas where both record strips are transparent (Fig. 1 b), and the total light passing through will represent the correlation product of the positive parts (if the transparent parts represent these) of the limiter outputs averaged over the time period represented by the length of the records. To obtain the complete correlation product the process must be repeated with records in which transparencies represent negative parts, and the result summed with that of the previous operation. To effect the varying time delay necessary to produce the cross correlation function the record of one signal is moved relatively to the other, thus producing changes in the light energy passing through the record pair. If the changes in light energy are recorded with photo-cells, a record of these changes plotted against relative record movement represents the cross correlation function. The effect of this relative movement can be produced by rotating an illuminating collimated light beam, with the two record strips stationary and parallel but some distance apart. Rotation of the light beam produces an effective time shift, and the output from a photo-cell acting as detector represents the cross correlation function. The signals may be passed continuously through the viewing area, and provided the time sweep of the collimated light beam is less than the time taken for the records to pass through, no information will be lost. In some applications it is necessary to provide cross correlation between all pairs of a number of continuously generated signals and to add the outputs of the individual correlations. This may be effected by the arrangement of Fig. 3, where instead of using a single collimated light beam, a long straight filament lamp L is placed at the axis of a parabolic mirror M so that a parallel light beam passes through the two similar moving records S1 and S2 (which are of the form shown in Fig. 3a). The light energy collected and measured by the photo-cell represents the total cross correlation effect of all signals correlated with each other. The lamp and reflector may be rotated in the horizontal plane to produce the effect of time delays. This system may not be a practicable possibility, so a system producing the same effect is explained wherein the space between the record strips is filled with a block of glass or plastic laminations, each lamina being coated so that light rays passing through the upper record strip pass through the block by a series of total internal reflections, to the lower. An arrangement is also explained where the bottom of the lamina block is silvered, so that the light rays are reflected back along these paths. This enables the lower record strip to be dispensed with, the photo-cells P being placed above the upperstrip S1. A system for producing cross correlation between one or more continuously generated signals and two reference signals of limited duration is indicated in Figs. 8 and 9. Two double channels are recorded side by side for each continuously generated signal on a film strip and a corresponding permanent record is formed around a drum of the sequence of reference signal records, half in one channel and half in the other. The continuously generated strip is pulled around a transparent shell concentric with the drum. Photo-cells around the cylinder collect light from the long filament light at the axis in a similar manner to that described above. In some applications the generated signals may contain rapid phase fluctuations which cause loss of correlation with the reference signal. The effect may be overcome by scanning the viewing length of the correlator of Fig. 9 with a collimated beam (Figs. 10 and 11, not shown). Frequency analysis of continuously generated signals may also be carried out by using a reference pattern having a continuously changing frequency characteristic (Fig. 12, not shown). Systems are mentioned for summing the correlation effects of both positive and negative parts of the continuously generated signals to produce the maximum correlation information (Fig. 13, not shown).